ELA Residual
Definition
The pixel-level absolute difference at a given location after re-compression. High residual means the pixel value moved substantially on re-compression, indicating the region had more information available to lose. Low residual means the region was already near the quantisation floor.
- Definition
- Per-pixel absolute difference after re-compression
- High value meaning
- More information available to lose
- Low value meaning
- Near the quantisation floor already
- Context
- JPEG compression artefact analysis
Common questions
Why does a low residual not necessarily mean a region is unedited?+
A region can reach a low residual simply because it was already heavily compressed in a prior save, for instance a flat sky or a smoothly shaded surface, so a low residual reflects compression history and image content as much as it reflects authenticity.
How does residual pattern help distinguish a spliced region from the rest of an image?+
A splice inserted from a source with a different compression history typically settles at a different residual level than the surrounding original content, creating a visible boundary in the residual map even though no single pixel value looks unusual on its own.
Related terms
- Compression Floor
- The stable low-residual state that a JPEG region approaches after many successive saves at the same quality. Once the DCT coefficients are...
- Discrete Cosine Transform (DCT)
- A mathematical transform that expresses a block of pixel values as a sum of cosine functions at different frequencies. JPEG applies the...
- Error Level Analysis (ELA)
- A technique that re-compresses a JPEG at a controlled quality setting and maps the pixel-level difference between the re-compressed and original images....
- Quantisation Step Size
- A quality-dependent divisor applied to each DCT coefficient before rounding. A larger step size (lower quality) discards more information and introduces more...
- Reference Quality
- The quality setting used to re-compress the image when computing ELA. The choice of reference quality controls the magnitude of all residuals...